1. Field of the Invention
The present invention refers to an integrated optocoupler with an organic light emitter and an inorganic photodetector, and in particular to a monolithically integrated CMOS optocoupler with an OLED light source.
2. Description of the Related Art
Optocouplers have been widely spread as technical means for galvanic isolation and/or electrical potential separation. This includes applications in car industry, consumer electronics, medical and measuring technology, data communication and the like. Optocouplers are used for electrical isolation of an input signal from a corresponding output signal and can be used as a substitution for relays and Reed relays (protective glass relays). They are advantageous because of their fast switching speed, higher reliability, better electrical isolation and, compared to conventional relays, drawbacks due to mechanical bonding and/or switching (so-called bounce effects) are avoided.
An optocoupler has a light emitter and a light receiver, which are optically coupled through a coupling medium and/or a light guide. Light-emitting diodes (LED), which emit e.g. infra-red light or red light, are often used as light emitters. Photodiodes, phototransistors, photothyristors, phototriacs, photo-Schmitt-triggers or photo-Darlington-transistors are e.g. used as light receivers. Light emitters and light receivers are connected to each other through an electrically non-conductive insulator.
The electrical isolation of circuit parts is necessary for potential separation, but also in order to avoid parasitic reactions. Potential separation is inter alia advantageous as a protection against hazards in medical appliances, but also in data communication (network/interface card) or as over-voltage protection. Parasitic reactions, which one would like to suppress with optocouplers, are for example noise in small signals or transients in engine control. Binary or analogue signals are transmitted in optocouplers.
An optocoupler is shown schematically in FIG. 10, a light emitter 20 converting an electrical signal, which is present at the input 10, into an optical signal 40. The optical signal 40 is transmitted from an optical outlet area 25, by means of a light guide 30, to an optical inlet area 55 of a light receiver 50. The light receiver 50 re-converts the optical signal 40 into an electrical output signal, which is present at an output 60. For potential separation, it is important that the light guide 30 electrically isolates the light emitter 20 from the light receiver 50, i.e. that the light guide 30 has a transparent dielectric material.
The market for optocouplers can be divided into two different main areas, on the one hand the simple optocoupler based on the classical photodiode and/or photo- (Darlington) transistor, on the other hand the fully integrated optocouplers with a CMOS read circuit (CMOS=Complementary Metal Oxide Semiconductor) for higher functions.
Photodiodes as possible light receivers and/or photodetectors 50 can be reproduced during a standard CMOS process at different p-n interfaces and FIG. 9 shows an example in an n-well CMOS process according to the state of the art. Formed in a p-doped substrate (p-type substrate) 910 is an n-doped well (n-well) 920, which has a p+-doped layer 930 on the side opposite the p-type substrate 910. As a final layer for the photodetector 50, the p-type substrate 910 has an oxide layer 940, which is followed by usual CMOS layers, such as e.g. an ILD layer 950 (ILD=Inter Layer Dielectric) and a IMD layer 960 (IMD=Inter Metal Dielectric). The oxide layer 940, the ILD layer 950 and the IMD layer 960 have a dielectric material and are translucent. Several p-n junctions are designated in FIG. 9 by diodes 925, 935 and 975.
Incident light beams 990 generate in the n-well 920 a pair of load carriers 985 of opposite loaded polarity, which is separated according to the polarity and causes an electrical signal. The photodetector 50 is comprised of the layers: p-type substrate 910, n-well 920, p+-doped layer 930. FIG. 9 shows furthermore a photodiode 975, which is comprised of a p-n junction of p-type substrate 910 and an n+-doped surface layer 970. The light signals 980 represent e.g. light reflected at the surface layer 970.
Known fully integrated optocouplers are based on a CMOS-based receiving and evaluating chip, as well as on an emitter (light emitter) 20 comprised of conventional (inorganic) light-emitting diodes (i.e. conventional LEDs), which have typically an optically close connection with the photodiode and/or with the photodetector 50. These two technologies use materials and processes that differ from each other. The standard CMOS process is mostly based on single-crystal silicon material, while conventional light-emitting diodes mostly use single-crystal III-V semiconductors. Therefore, the elements photodetector 50 and light-emitting diode as light emitter 20 cannot be produced monolithically, but can exclusively be integrated in a hybrid way with each other.
Conventional light-emitting diodes comprised of inorganic semiconductors, such as e.g. GaAs and related III-V semiconductors have been known for decades. The basic principle of such light-emitting diodes is that by applying an electric voltage electrons and holes are injected into a semiconductor and combine through radiation in a recombination zone during light emission.
As an alternative to inorganic light-emitting diodes, light-emitting diodes based on organic semiconductors have in recent years achieved large progresses. For example, an organic electroluminescence presently experiences great attention as a medium suitable for displays. Organic light-emitting diodes have a series of organic layers with a thickness typically in the range of 100 nm, which is inserted between an anode and a cathode. Glass is often used as a substrate, on which a transparent electrically conductive oxide, such as e.g. indium-tin oxide (ITO=Indium-Tin Oxide), is applied. Then follows the series of organic layers, which has hole-transporting material, emitting material and electron-transporting material. A metallic cathode usually follows then.
FIG. 8 shows organic light-emitting diodes (OLED) according to the state of the art, at the left being shown a so-called bottom emitter 810 and at the right a so-called top emitter 820.
In the case of the bottom emitter 810, a transparent electrode 814, a series of organic layers 816 and a second electrode 818 are deposited on a substrate 812. The transparent electrode 814 is electrically connected through a first contact 815 and the second electrode 818 through a second contact 819. In the series of organic layers 816, an electric signal at the first contact 815 and the second contact 819 is converted into a light signal 40 which is radiated mainly downward in the representation shown here.
As already mentioned above, the following materials can be used. For the transparent electrode 814, e.g. ITO can be used, while for the second electrode 818 a metal is often used and the series of organic layers 816 has e.g. a hole-transporting material, an emitting material and an electron-transporting material. An upward radiation is prevented in the case of the bottom emitter 810 in that the second electrode 818 has an opaque material. In order for the substrate 812 not to prevent light propagation, glass is typically used as transparent material.
The top emitter 820, which is represented at the right side in FIG. 8, has an accordingly reversed series of layers. The second electrode 818 which has an opaque material is deposited on a substrate 822. Then follows the series of organic layers 816, followed by the transparent electrode 814, which has a transparent material (e.g. ITO). The transparent electrode 814 is, in turn, electrically connected through the first contact 815 and the second electrode 818 through the second contact 819. In this case too, an electrical signal at the first contact 815 and the second contact 819 is converted in the series of organic layers 816 into a light signal 40, which because of the opacity of the second electrode 818 is mainly radiated upward in the representation shown.
Irrespective of the selected representation, the bottom emitter 810 radiates the light signal 40 mainly through the substrate 812, while the top emitter 820 radiates in a direction away from the substrate 822. The light signal 40 in FIG. 8 indicates a main radiation direction. The light generated in the series of organic layers 816 however also propagates along the series of organic layers 816 or along the transparent electrode 814 and, if no lateral protection is present, is also partly radiated sidewise.
Optocouplers with an integration of both inorganic emitters (i.e. the light emitter 20), which are usually based on III-V semiconductors, and inorganic detectors (i.e. the photodetector 50), such as e.g. CMOS photodiodes are known. Optocouplers with the integration of both organic emitters and organic detectors are also known, which are described for example in DE 10061298 A1 or DE 10103022 A1. Furthermore, photodetectors for picture recorders are known, which are implemented in a silicon handle wafer of an SOI substrate (SOI=Silicon On Insulator) (inter alia U.S. Pat. No. 6,838,301 B2).
Since conventional LEDS, as has been said, primarily use III-V semiconductors and the detector circuit (i.e. the photodetector 50 and the activating circuit) is mostly based on silicon, both elements cannot be produced on the same substrate and an integration therefore proves difficult. A possible hybrid integration in optocouplers, such as known for example from the state of the art, necessitates in principle a higher manufacturing cost and, as a matter of fact, does not allow a general price regression with large numbers of units. Furthermore, because of the hybrid structure, the necessary reliability for car applications can be achieved only at an extremely high cost.